Ethanol for industrial use is conventionally produced from petrochemical feed stocks, such as oil, natural gas, or coal, from feed stock intermediates, such as syngas, or from starchy materials or cellulose materials, such as corn or sugar cane. Conventional methods for producing ethanol from petrochemical feed stocks, as well as from cellulose materials, include the acid-catalyzed hydration of ethylene, methanol homologation, direct alcohol synthesis, and Fischer-Tropsch synthesis. Instability in petrochemical feed stock prices contributes to fluctuations in the cost of conventionally produced ethanol, making the need for alternative sources of ethanol production all the greater when feed stock prices rise. Starchy materials, as well as cellulose material, are converted to ethanol by fermentation. However, fermentation is typically used for consumer production of ethanol, which is suitable for fuels or human consumption. In addition, fermentation of starchy or cellulose materials competes with food sources and places restraints on the amount of ethanol that can be produced for industrial use.
Ethanol production via the reduction of alkanoic acids and/or other carbonyl group-containing compounds has been widely studied, and a variety of combinations of catalysts, supports, and operating conditions have been mentioned in the literature. During the reduction of alkanoic acid, e.g., acetic acid, other compounds are formed with ethanol or are formed in side reactions. These impurities limit the production and recovery of ethanol from such reaction mixtures. For example, during hydrogenation, esters are produced that together with ethanol and/or water form azeotropes, which are difficult to separate. In addition when conversion is incomplete, unreacted acid remains in the crude ethanol product, which must be removed to recover ethanol.
Alkanoic acids, such as acetic acid, used in the production of ethanol are commonly synthesized by the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329, incorporated herein by reference in its entirety. The carbonylation catalyst contains rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms. Likewise, because the nature of the halide promoter is not generally critical, a large number of suitable promoters, most of which are organic iodides, may be used. Most typically and usefully, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved.
Carbonylation processes in the presence of iridium catalysts are also known and are described, for example, in U.S. Pat. No. 3,772,380 and in U.S. Pat. No. 5,672,743, the entireties of which are incorporated herein by reference. UK patent GB 1276326 describes the preparation of monocarboxylic acids and their esters by carbonylation of alcohols, halides, ethers, esters or 1,2 epoxy aliphatic compounds in the presence of rhodium or iridium catalysts, halogen promoters and water or an alcohol, ether or ester.
Both Iridium and Rhodium based carbonylation catalysts suffer in that may tend to form impurities such as propionic acid, typically necessitating multiple columns to form a suitable acetic acid product having desired purity. Significant capital and operational costs are also incurred by the necessity of operating a large distillation column (commonly referred to as a “Heavies” or “Heavy Ends” column) to remove low levels of high boiling point impurities, with propionic acid being the major component.
The need therefore exists for new processes for mitigating impurities, such as propionic acid, that are commonly formed in the synthesis of acetic acid. The need also exists for improved processes for synthesizing ethanol from a crude product obtained by reducing alkanoic acids, such as acetic acid, and/or other carbonyl group-containing compounds.